1. Field of the Invention
The present invention relates to heaters, and more particularly, to a heater for a liquid crystal display and backlight.
2. Description of Related Art
A liquid crystal display (LCD) is commonly used in laptop computers and other devices that require a visual output to display information such as automotive and aircraft dashboard displays. A typical LCD comprises a digital display that consists of two sheets of glass separated by a sealed-in, normally transparent, liquid crystal material. The liquid crystals are actually rod-shaped molecules that flow like liquid and bend light. The outer surface of each glass sheet has a transparent conductive coating, such as indium tin oxide, with the viewing-side coating etched into character forming segments that have leads going to the edges of the display. A voltage applied between front and back electrode coatings disrupts the orderly arrangement of the molecules, darkening the liquid enough to form visible characters even though no light is generated.
Reflective screens used in many consumer appliances and some lightweight laptops require external light such as a desk lamp. A LCD that may have to be viewed under both bright lighting conditions and/or in darkness will include a light source, some configurations referred to as a backlight or sidelight, for background lighting of the LCD. Many LCDs will use a translucent reflector with the light behind it, thereby making the background brighter and the characters sharper. This light source must provide a long service life, a high uniform light yield, and emit a constant color over temperature when colored LCDs are used. A fluorescent lamp is an example of a light source that generally meets these requirements. However, a disadvantage of fluorescent lamps is that their illumination level is a function of temperature. A fluorescent lamp usually provides the maximum light yield at ambient temperatures of approximately 40.degree. to 50.degree. C. In the case of low ambient temperatures, such as below 10.degree. C., fluorescent lamps often fail to provide adequate lighting.
To overcome the shortcomings of fluorescent lamps at low temperatures, incandescent light sources may also be provided for lighting the LCD. An incandescent lamp extends the low temperature operation of the LCD, for example to minus 50.degree. C. The incandescent lamp may also serve as a heat source for the fluorescent lamp, thereby raising its operating temperature to a more efficient level for light output. Unfortunately, incandescent lamps provide only a low level of light in comparison with fluorescent lamps and, due to their predominantly point-source nature, provide non-uniform illumination as well as heat.
Other methods have been devised to extend the low temperature operation of the fluorescent lamp. For example, one method is to coat the lamp with a temperature-controlled foil-type heating element. Another method is to provide the lamp with a higher voltage during low temperature operation in order to obtain a higher level of light output. Both of these methods have drawbacks, as the foil-type heating element fails to provide adequate heat for the LCD display, as discussed below, and the operation of the lamp at a higher voltage reduces the life of the lamp.
The functioning of the LCD is also temperature dependent. A twisted nematic, for example, is a type of liquid crystal used in LCDs that are often installed in vehicle dash displays. The twisted nematic LCD consists of a matrix array of liquid crystal picture elements and typically a corresponding backlight for illuminating the elements. When the driving voltages are applied across the liquid crystal material, the twisted nematic liquid crystals tend to align themselves to provide a desired image to the viewer. Below a predetermined temperature, however, the twisted nematic liquid crystal material does not behave in a desired manner. Therefore, the liquid crystal material must be heated to a sufficient temperature, usually above minus 10.degree. C., in order to achieve satisfactory performance.
Furthermore, if the LCD is exposed to drastic changes in atmospheric conditions, such as on a fork lift transitioning from a warm ambient temperature to a commercial freezer storage unit, condensation rapidly forms on the inside of the LCD panel due to vaporized water droplets in the ambient air within the LCD. This significantly degrades the readability of the LCD. Thus, a heater for the LCD is generally used to heat the liquid crystal material to a sufficient temperature and also to reduce the amount of condensation within the LCD.
LCD heaters, such as a conductive indium tin oxide (ITO) coating deposited on a transparent glass substrate or rear polarizer, are well known in the art. The ITO coating is typically energized by way of a pair of parallel buss bars aligned along a planar surface of the ITO with the buss bars being made of either silk-screened conductive epoxy or deposited metal. However, these methods of forming the buss bars have a number of disadvantages. For example, silk-screened conductive epoxy buss bars exhibit less than desirable conductance and this increases the required time period for warming up the corresponding display. Deposition of the metal onto the ITO layer is difficult, expensive, time consuming, and can result in a burned out buss bar or thick electrical connections that are stress concentrators when sandwiched between the plates. It is also difficult and time consuming to attach a conductive wire to the buss bar. In addition, if the ITO layer is scratched, the entire LCD cell must immediately be replaced. Finally, the entire process of providing an ITO layer along with buss bars is difficult and time consuming and is not easily applied to an off-the-shelf LCD module requiring a heater.
Alternatives to the LCD are available. An example would be the electroluminescent display that provides adequate performance down to temperatures as low as minus 30.degree. C. However, this type of display is much more costly than the typical LCD, and is therefore undesirable for many applications.
Accordingly, it would be desirable to provide a heater for the LCD and backlight that is inexpensive and easy to install or remove as part of an LCD assembly, in an existing off-the-shelf LCD, or even for LCDs that are currently installed in devices. The heater would thus extend the low temperature operation of the backlight as well as the LCD and its accompanying electronics, help to remove condensation that may form on or within the LCD display, and extend bulb life by reducing the low temperature operating time.